1. Field
The present application relates to optimizing of communication signal.
2. Description of the Related Art
A device connected to a network that complies with the IEEE1394.b standard is operable in one of a plurality of different transfer speed modes. For example, a device may be operable in one of S400, S800, S1600, S3200 modes. Such a device is connected to a network by a cable. In these modes, devices generate output signals having amplitudes (output amplitudes). The maximum value of the output amplitudes is 800 mV and the same in each mode. However, these modes have different maximum transfer speeds and different tolerable minimum values for the output amplitudes. In such a network, power consumption of the device must be reduced while enabling stable communication between devices.
In the S400 mode, the maximum transfer speed is 500 Mbps, and the minimum value for the output amplitude is 300 mV. In the S800 mode, the maximum transfer speed is 1 Gbps, and the minimum value for the output amplitude is 350 mV. In the S1600 mode, the maximum transfer speed is 2 Gbps, and the minimum value for the output amplitude is 475 mV.
The maximum transfer speeds and minimum values of the output amplitudes for the IEEE 1394.b standard are higher than that for the IEEE 1394.a standard. Accordingly, in a network that is in compliance with the IEEE 1394.b standard, the power consumption tends to be greater than that of a network that is in compliance with the IEEE 1394.a standard.
To operate devices connected to a network in different modes, the minimum value of the output amplitude for a transmission circuit must be the same in each device. In such a case, to ensure stable data transmission, the minimum value of the output amplitude for each device must be set in accordance with the device having the highest minimum output amplitude.
Accordingly, when transmitting data from a device having a high transfer speed to a device having a low transfer speed, the minimum output amplitude becomes unnecessarily high. This results in unnecessary power consumption.
The standardized maximum length of a cable, which uses copper wires, for connecting devices is 4.5 m. However, a device used in a vehicle may require a longer cable. In such a case, to perform stable communication between devices, the output amplitude value must be set at a significantly higher level. As a result, for other devices that are connected by standardized cables, unnecessary power is consumed.
Further, if the output amplitude value is set to be high, this would increase the radiated electromagnetic waves and produce electromagnetic wave noise.
Japanese Laid-Out Patent Publication No. 2003-46384 describes an output circuit for setting the amplitude of an output signal to a desired value regardless of the resistance in a transmission path. However, the publication does not teach the reduction of power consumption in a device that operates in compliance with the IEEE 1394.b standard.